I. Field of the Invention
The invention relates to an apparatus for measuring flow vectors in gas currents.
II. Description of Related Art
U.S. Pat. No. 3,941,477 discloses an optical device in which a light beam emitted from a light source is divided into two partial beams, each of which is focused in the measuring volume. Solid particles contained in the flow and traversing a focusing point, flash up, and said flash up is detected by an analyzer. If the flow vector of a particle is in alignment with a straight line traversing both focusing points, there is produced a start pulse at the first focusing point and a stop pulse at the second focusing point. From the time interval between said pulses, one may detect the flow rate; i.e., the vector amount. The measuring process involved with said two focus-processes is quite time-consuming. In fact, only such flow vector components which are situated in a plane extending vertically to the optical axis of the system may be measured. Said plane must be determined by a series of measurements. The vector component extending in parallel to the optical axis cannot be determined.
British Patent No. 2,109,548 describes a further development of the stated double-focus process. According to said British patent, it is also possible to measure a flow vector component extending in parallel to the optical axis of the system so that the flow vectors may be determined completely as to amount and direction. In this process, called the three component process, a multicolored laser beam is split into two components of different colors. Each of said components is split up into partial beams which are polarized vertically to each other. Thus, there are formed four partial beams in total of which two always intersect in the measuring volume and form a focusing point. One focusing point is situated on the optical axis of the system. By turning an image rotating prism, the position of the other focusing point may be easily changed. In order to evaluate the flashes produced by the flowing particles, the radiation of said flashes is selected subject to polarization and color in order to generate the start and stop pulses.
Not only in case of the double-component process, but also in the three-component process, the distance between the focusing points is dictated by the beam splitter employed for generating, from the laser beam, two differently colored partial beams. The beam splitter being a Rochon prism, may be exchanged, if necessary. In case of the three-component process, it is additionally necessary to adjust a deflecting mirror containing a focusing diaphragm and having holes to allow the passage of the two differently colored laser beams and to reflect the returning light beams in order to laterally guide them out of the optical system. The measuring time may be reduced considerably if the distance of the focusing points in the measuring volume is conformed with the flow turbulences. However, in the known devices, such changes of the beam distance are accompanied by a considerable waste of time for assembly and adjustment operations which cannot be made during a measuring campaign.